1. Field of the Invention
The present invention relates to a zoom lens system and an electronic image pickup apparatus using the same, and particularly, to an electronic image pickup apparatus such as a video camera and a digital camera, in which, small size is realized.
2. Description of the Related Art
In recent years, replacing a silver-salt film camera, a digital camera in which, an object is photographed by using a solid image pickup element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) has become a mainstream. Furthermore, there are several categories of digital cameras in a wide range, from a high-function type for professional use to a compact popular type.
A user of the popular type digital camera seeks to enjoy photography readily, anywhere at any time with a wide range of scenes. In light of this, a small size camera, particularly a slim digital camera which can be accommodated easily in a pocket of clothes or a bag, and carried conveniently has been preferred. On the other hand, while a zooming ratio of about 3 for a digital camera of a compact type has been common, a camera of a higher zooming ratio than the conventional one for widening a photography area has been sought.
Recently, a so-called collapsible barrel in which, when the camera is in use, the lens barrel is pushed out from a camera body, and is accommodated inside the body while carrying the camera has been generally used. Therefore, it is important to structure the zoom lens optical system upon taking into consideration the slimming of the lens barrel when collapsed. As a conventional technology, a compact zoom lens system which is a four-unit zoom lens system including in order from an object side, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, and a fourth lens unit having a positive refracting power has been known by Japanese Patent Application Laid-open Publication No. 2006-23531.
In the zoom lens system disclosed in Japanese Patent Application Laid-open Publication No. 2006-23531, the first lens unit includes not more than two lenses, and the second lens unit includes not more than three lenses, which is advantageous for making small the size of the zoom lens system in a radial direction. Moreover, with less number of lenses, the structure is advantageous for making the size small. In the above-mentioned prior art, an idea has been devised for reducing (making small) a size in a direction of thickness of lens units, by reducing the number of lenses in each lens unit.
Particularly, since a height from an optical axis of off-axis rays becomes is increased in the first lens unit and the second lens unit, when an attempt is made to secure a necessary amount of an edge thickness of a lens, an axial edge thickness tends to increase remarkably. Furthermore, when the number of lenses in the first lens unit and the second lens unit increases, since an entrance-pupil position becomes distant as viewed from the object side, the height of the off-axis rays passing through the first lens unit and the second lens unit becomes increasingly higher, and a longitudinal thickness for securing the edge thickness is required to be even more. Naturally, the amount of increase in the number of lenses increases the longitudinal thickness.
Consequently, with the increase in the number of lenses, the size in the radial direction and thickness on the optical axis of this lens unit is increased to be more than necessary. Even in the collapsed state, the lens barrel cannot be made to be sufficiently compact. From this view point, forming the first lens unit and the second lens unit by less number of lenses is advantageous for making the lens barrel compact.